The Antiphospholipid Syndrome is characterized by naturally occurring antiphospholipid antibodies and increased risk of pregnancy loss or a variety of obstetric complications. Antiphospholipid antibodies are directed against phospholipid-dependent antigens, phospholipid-binding proteins, or a combination of these. There is little information concerning the relevance and relationship of particular antibody specificities to pathophysiology. Considerable data support the proposal that the placental trophoblast is a primary cellular target of antiphospholipid antibodies. The hypothesis is that antiphospholipid antibody-induced pregnancy loss occurs through the binding of a select subgroup of antibodies directly with trophoblast resulting in detrimental effects on placental development. Some of these effects involve thrombosis on the trophoblast surface, but most are non-thrombotic and result in incomplete trophoblast differentiation. A prediction, based on preliminary data, is that the most relevant antibodies will be against phosphatidylserine-dependent antigens. To test this hypothesis, the following Aims will be addressed. Aim 1 will investigate the heterogeneity of antiphospholipid antibodies within individual patients. This Aim is designed to determine within individual pregnancy loss patients the spectrum of reactivities against relevant phospholipids and phospholipid-binding proteins. Individual antiphospholipid antibodies will be purified based on antigenic specificity and characterized. Aim 2 will clone antiphospholipid antibodies from individual patients using single cell PCR techniques. The goal is to clone each relevant antibody present in a minimum of 3 individual patients. The cloned antibodies will be characterized. Aim 3 will investigate the relationship between antibody specificity and biological activity against trophoblast. Reagents prepared in Aims I and 2 will be tested for reactivity against trophoblast. These reactivities will include binding to normal human placental tissue, binding to trophoblast undergoing differentiation in vitro, inhibition of trophoblast invasion, inhibition of intertrophoblast fusion, and induction of apoptosis. This will be the first study to thoroughly investigate the spectrum of antiphospholipid antibodies in individual patients and correlate those data to effects on placental trophoblast.